1. Field of the Invention
The present invention generally relates to couplers for coupling optical radiation into and out of an optical fiber.
2. Description of Related Art
Optical fibers have by far the greatest transmission bandwidth of any conventional transmission medium, and therefore optical fibers provide an excellent transmission medium. An optical fiber is a thin filament of drawn or extruded glass or plastic having a central core and a surrounding cladding of lower index material to promote internal reflection. Optical radiation (i.e. light) is coupled (i.e. launched) into the end face of an optical fiber by focusing the light onto the core. For effective coupling, light must be directed within a cone of acceptance angle and inside the core of an optical fiber; however, any light incident upon the surrounding cladding or outside of the acceptance angle will not be effectively coupled into the optical fiber.
It is a difficult task to couple light into the central core of an optical fiber due to its small size and acceptance angle, particularly if the optical fiber is a single mode optical fiber. A typical single mode fiber has a core diameter of only 10 microns and an acceptance angle of only 10xc2x0. Single-mode fibers, which are designed to transmit only single-mode optical radiation, are widely utilized for telecommunications applications. Multimode optical fibers have a larger cross-section and a larger acceptance angle than single-mode fibers. For example, a typical multimode fiber has a core diameter of 50 microns and an acceptance angle of 23xc2x0. Because any optical radiation outside the core or acceptance angle will not be effectively coupled into the optical fiber, it is important to precisely align the core with an external source of optical radiation.
One conventional practice for making a fiber-pigtailed transmitter is to assemble an edge-emitting laser diode, an electronics circuit, a focusing lens, and a length of optical fiber and then manually align each individual transmitter. To align the transmitter, the diode is turned on and the optical fiber is manually adjusted until the coupled light inside the fiber reaches a predetermined level. Then, the optical fiber is permanently affixed by procedures such as UV-setting epoxy or laser welding. This manual assembly procedure is time consuming, labor intensive, and expensive. Up to 80% of the manufacturing cost of a fiber-pigtailed module can be due to the fiber alignment step. The high cost of aligning optical fiber presents a large technological barrier to cost reduction and widespread deployment of optical fiber modules.
One single-mode fiber has a cylindrical glass core of about 10 microns in diameter surrounded by a glass cladding with a circular outer diameter of about 125 microns. In some connections, slight variations in dimensions can drastically affect coupling efficiency, and therefore some optical fiber manufacturers carefully control the fiber""s tolerances. For example, in a splice connection between two optical fibers, a large loss in the transmitted signal can occur if the two inner cores fail to align precisely with each other. For example, if the cores of two 10-micron single-mode fibers are offset by only 1 micron, the loss of transmitted power through a splice is about 5%. Therefore, to reduce coupling losses, manufacturers maintain cladding diameter tolerances within the micron to sub-micron range. For example, Corning Inc. specifies the tolerance of its optical fibers as 125xc2x11 micron.
In order to provide passive alignment of optical fibers, various alignment techniques have been reported based on precisely etched holes on a wafer. For example, in Matsuda et al. xe2x80x9cA Surface-Emitting Laser Array with Backside Guiding Holes for Passive Alignment to Parallel Optical Fibersxe2x80x9d, IEEE Photonics Technology Letters, Vol. 8 No. 4, (1996) pp. 494-495, a research group at Matsushita in Japan performed an experiment in which a shallow guiding hole on the backside of a back-emitting vertical cavity surface emitting laser (VCSEL) wafer is etched to a depth of 10 to 15 microns and a diameter of 130 microns. A multi-mode fiber stem 125 microns in diameter is inserted into the guiding hole with a drop of epoxy for passive alignment to the VCSEL. This group reported an average 35% coupling efficiency at 980 nanometers. The large core diameter of multi-mode fibers (e.g. 50 microns) allows this approach to be suitable for coupling light into multi-mode fibers; however the lack of a light-focusing mechanism prevents use of this method with single-mode fibers.
U.S. Pat. No. 5,346,583 to Basavanhally discloses a substrate having at least one lens formed on a first surface. An optical fiber guide is etched on a second surface of the same substrate, opposite the first surface. The optical fiber guide is used to mount an optical fiber on the second surface such that the central axis of the optical fiber is substantially coincident with the central axis of the lens, thereby giving the desired alignment. Fused silica and silicon are two common substrate materials. If the substrate material is fused silica (or glass), the fiber guide etch rate is very slow (typically 0.3 micron per minute or less) and as a result it is impossible to obtain fiber guides of sufficient etch depth, which is necessary to obtain precise angular alignment to single mode fibers. According to the method described in the patent, etching is to stop before it reaches the first surface where the lens resides. At the bottom of the etched fiber guide, the surface is typically neither smooth nor flat, which could cause scattering and reflection loss if the refractive index of the substrate material is different than that of the optical fiber core (approximately 1.5).
U.S. Pat. No. 5,195,150 to Stegmueller et al. discloses an optoelectronic device that includes a substrate that has a recess for receiving a plano-convex lens and a recess on the other surface of the substrate aligned with the lens to receive an end of an optical fiber. The device disclosed by Stegmueller is subject to the same problems as the device disclosed in the Basavanhally patent.
In order to overcome the limitations of prior art optical fiber couplers, the present invention provides a multilayer optical fiber coupler for coupling optical radiation between an optical device and an optical fiber, including a first layer that has a fiber socket formed by photolithographic masking and etching to extend through said first layer, and a second layer bonded to the first layer. A multilayer optical fiber coupler is described that has a vertical through hole (a xe2x80x9cfiber socketxe2x80x9d) in a first layer that precisely aligns an optical fiber with an optical focusing element formed in the second layer. A method for forming the fiber couplers is described herein that can advantageously utilize semiconductor processing techniques including photolithography and dry etching to fabricate the couplers. The precision of the fiber socket structure allows single mode optical fibers to be passively aligned, and is also useful for aligning multimode optical fibers.
In one embodiment, a first layer, typically comprising substantially single-crystal silicon, is deep-etched using a suitable etching process such as silicon Deep Reactive Ion Etching (DRIE), which is a dry etching process, to form an array of fiber sockets that extend through the first layer. A second layer is formed to provide a corresponding array of optical focusing elements. The first and second layers are aligned using alignment fiducials and permanently bonded together, so that the fiber socket in the first layer precisely aligns the core of the optical fiber with the optical focusing element in the second layer. The bonded structure is then diced to form a plurality of separate couplers or arrays of couplers. An optical fiber is affixed into each fiber socket by any suitable means, such as an optical epoxy.
In order to provide precise, passive alignment of the optical fiber within the fiber socket, the fiber socket is formed to be only slightly larger than the fiber diameter. Single-crystal silicon is particularly useful to form the fiber sockets because silicon DRIE techniques, which are a type of dry etching, have been developed recently as a result of advances in microelectromechanical system (MEMS) research, which allow vertical holes to be etched at high speeds (up to 10 micron/minute at present) with less than 1 micron vertical variation in hole diameter (i.e. xc2x10.5 micron). In one embodiment, the deep-etching process uses high definition photolithography and an appropriate high etch selectivity mask to create precisely-dimensioned fiber sockets. These fiber sockets then receive precisely-dimensioned optical fibers, thereby accurately aligning the optical fibers within the fiber socket. The fibers are held in place by epoxy or another suitable adhesive.
In one embodiment the second layer comprises borosilicate glass such as PYREX, which is advantageous for several reasons. The glass can be strongly and conveniently bonded to silicon by anodic bonding, which is a dry bonding process. The thermal expansion coefficient of borosilicate glass matches well with that of silicon, which provides a durable and reliable structure. Furthermore, the index of refraction of borosilicate glass approximately matches the index of refraction of the core of the optical fiber, which is the light transmitting section of the fiber, and therefore an optical epoxy can be used that also approximately matches the index of refraction of the optical fiber. In such an embodiment, the glass, epoxy, and optical fiber form a natural index-matched system, eliminating the need for polishing and anti-reflection coating the end face of the optical fiber which are current fiber optic industry practices, and resulting in further cost savings. Due to the index matching in some embodiments, optical radiation advantageously propagates substantially loss-free through the fiber end face, epoxy, and the adjacent surface of the second layer.
Due to the fiber sockets formed to extend through the silicon layer, a large number of single mode optical fiber couplers can be made on the wafer level with very low cost. One cost advantage is attributed to the batch microfabrication process and the elimination of the need to actively align the fiber. For example, assuming a 4-inch integrated wafer and a 1 mmxc3x971 mm die size, about 7800 fully-integrated chips can be obtained by dicing the wafer. This approach allows optical couplers as well as other devices disclosed herein to be manufactured with the same kind of economies of scale as the silicon electronics industry, since the cost of the processing steps are shared by all the individual chips.
The optical fiber couplers are rugged and compact, and can be used in a variety of applications. The fiber couplers can be implemented in a wide variety of embodiments; for example the optical couplers may be incorporated with other devices such as VCSELs.